1. Field of the Invention
The invention relates to method for manufacturing ceramic materials. In particular, this invention relates to a hybrid method for firing ceramics involving microwave and conventional heating, and more particularly to a method for efficiently controlling the heating rate by separately controlling the proportions of microwave and conventional radiative/convective energy.
2. Discussion of the Related Art
Conventional heating used in the manufacturing of ceramic materials typically comprises radiative gas or electric resistance heating. Utilization of conventional radiative/convective heating typically results in a thermal differential within the ceramic body, due to the fact that radiant heating is applied only to the surface and it relies on thermal conductivity of the ceramic body, typically poor, to effect the temperature beneath the surface and into the interior of the piece. In other words, conventional heating involves heat transfer that is predominantly achieved by radiation or convection to the surface followed by conduction from the surface into the interior of the ceramic body. If a core-surface thermal differential develops that is too great, internal cracking and distortion of the ceramic body can occur. Fast firing further exacerbates this problem of poor heat transfer, and ultimately cracking. Additionally, the presence of a core-surface thermal gradient can also result in uneven sintering, specifically surface sintering prior to, and at a faster rate than, interior sintering. As a result, the ceramic body may exhibit non-uniform properties. Undesirable solutions involve reducing the rate of heating or allowing lengthy holds at certain temperatures. Each of these undesirable solutions allows heat energy to be conducted into the core of the ceramic body, which in turn, allows the temperature of the core of the ceramic body to xe2x80x9ccatch upxe2x80x9d with that of the surface, thereby minimizing the surface/core temperature differential. In summary, the theoretical limits of conventional radiative or convective heating typically result in slow heating rates for all ceramic bodies, the exception being ceramic bodies exhibiting small dimensions.
Microwave heating of ceramics has alternatively been successfully used to fire ceramic bodies. In comparison with conventional heating, microwave heating involves depositing energy directly within the ceramic body and involves a volumetric heating mechanism. Stated differently, the utilization of microwave energy involves delivering a uniform application of the energy to the entire cross section of the ceramic article, rather than to the article surface. Although microwave heating of ceramic bodies is much faster than conventional radiant heating because of this volumetric heating, it, like radiative heating, results in the ceramic body exhibiting a thermal differential; albeit an opposite thermal differential with the core of the ceramic body exhibiting a higher temperature than that of the surface. Specifically, as the ceramic materials, typically poor absorbers of microwave energy at low to intermediate temperatures, are heated with microwaves at high temperatures, the interior of the ceramic body very rapidly begins to absorb substantial amounts of microwave energy; this effect is known as thermal runaway. Although the surface is heated along with the core of the ceramic body, the surface rapidly loses much of its heat energy to the surroundings, typically cooler than the average ceramic material temperature. As the core starts to preferentially absorb the microwave energy this thermal runaway phenomenon becomes self-propagating. Simply stated, as the temperature of the ceramic body increases, the heat losses become greater, and the magnitude of the core-surface thermal differential increases, again leading to thermal stress within, and ultimately cracking of, the ceramic body.
In addition to heat losses from the surface of the ceramic body, non-uniformity of the microwaves within the kiln and non-uniform material properties of the ceramic leading to differential absorption of the microwave energy, contribute to this thermal differential due to microwave heating.
Hybrid microwave/conventional heating or microwave assisted heating has been proposed as an alternative to overcome the problems of conventional radiative and microwave-only heating. In microwave assisted heating involving both microwaves and radiative/convective heating, the volumetric heating provided by the microwaves heats the components, while the conventional heating radiative/convective provided by gas flame or electric resistance heating elements minimizes heat loss from the surface of the components by providing heat to the surface and its surroundings. This combination or hybrid heating can result in heating that avoids thermal profiles associated with conventional and microwave-only firing. As a result, thermal stresses are reduced and or minimized and thus the ceramic bodies can be heated more rapidly.
Typically, control of these microwave assisted firing methods involves placing a thermal measurement device on the surface to control the conventional firing rate and a second thermocouple into the core of the ceramic to control the microwave energy input. Experience with this method of control has revealed that the two control points are in too close of a proximity to each other to be effective and to result in the stable operation of the two control systems. The effects of the conventional firing are often picked up by the internal temperature measurement system, while the surface temperature measurement often registers effects of the microwave energy input. As a result, the two independent control systems become unstable and often result in an operation where one of the energy inputs, microwave or conventional, is manually controlled while the other is controlled automatically, as required, to maintain the desired heating rate or thermal profile; an inefficient control means, at best.
A slight variation on this microwave-assisted ceramic firing standard control method is disclosed in PCT Application WO 95/05058. This reference discloses a method of independently controlling the quantities of heat generated in the ceramic body by the microwave energy and radiant heat by measuring the ambient temperature within an enclosure containing the ceramic body. Based on, and in response to, this ambient temperature measurement, the heat generated in the ceramic body is controlled by one or both of the microwave energy or radiant heat. Although this control method is an improvement of the conventional control method, the mixing of kiln gases may not be uniform enough to accurately predict the ceramic body surface temperatures, thus reducing the effectiveness of the method. Further, many of the chemical reactions that occur within the ceramic body take place at temperatures low enough that radiant heat transfer is not a primary means of heat transfer from the ceramic body to the inside surfaces of the kiln where the kiln ambient temperatures are measured.
Accordingly it is an object of this invention to provide a method of, efficiently and effectively controlling the microwave and conventional radiative/convective energy utilized in the heating of ceramics that overcomes the shortcomings of the aforementioned hybrid microwave energy-conventional heating of ceramics.
The firing method of present invention comprises placing a ceramic article in a microwave heating apparatus having a microwave cavity and subjecting the ceramic material to an amount of heat energy by irradiating it with electromagnetic microwave radiation. The microwave power to which the ceramic is subject to is varied over time according to a predetermined microwave power-time or temperature-time profile. This predetermined microwave power-time or temperature-time profile includes periods of constant power output, periods where the power is ramped from a low output to a higher output, and periods where the power output is ramped from a high output to a lower output. Additionally, one embodiment of the method involves continuously measuring the ceramic article""s core and surface temperatures and adjusting the amount of conventional radiative/convective heat in response to the difference between the two measured temperatures. The conventional heat is adjusted to a level such that the surface temperature is caused to be at a temperature whereby the thermal differential between the surface temperature and the core temperature is maintained at an acceptable level. The firing of the ceramic article utilizing both microwave power and the conventional heat is continued until that hold temperature at which the ceramic article is typically held for a period of time to complete the sintering of the article, the so-called sintering soak or hold temperature.